University of Crete HEP Seminars


Super-Planckian scalars?

Speaker: Lukas Witkowski
Institution: University of Crete
Time: Tuesday 26 September 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Scalar fields with super-Planckian field ranges play an important role in models of large-field inflation and are also employed in cosmological solutions to the electroweak hierarchy problem. One difficulty in constructing such models is that for super-Planckian field values quantum-gravitational effects cannot be ignored. In particular, ultraviolet physics is expected to constrain the field range over which scalars can exhibit a monotonic potential, which in turn has consequences for models of inflation. To study theories of super-Planckian scalars some knowledge of ultraviolet physics is thus necessary. String Theory is a candidate theory of Quantum Gravity and a suitable framework for addressing this question. Another approach is based on the Weak Gravity Conjecture an as yet unproven theorem regarding consistent theories of Quantum Gravity. In this talk I want to report on recent progress in the study of super-Planckian scalars. In particular, I will explain how the existence of super-Planckian scalars is increasingly called into question by both String Theory and the Weak Gravity Conjecture.

Holographic RG flows on curved manifolds

Speaker: Lukas Witkowski
Institution: University of Crete
Time: Thursday 28 September 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Holographic RG is based on the idea that the radial coordinate of a spacetime with asymptotically AdS geometry can be identified with the RG flow parameter of the boundary field theory. Holographic RG flows have been studied intensively for the case of field theories defined on a flat background manifold. In this talk I will consider the case where the background spacetime for the field theory is curved. In particular, I will consider the cases of field theories defined on a sphere as well as dS and AdS spacetimes. In this setting I will explore solutions that have exotic properties from a RG point of view. There is also tantalising evidence for the existence of first-order phase transitions triggered by a change in curvature.

Quasinormal modes of a strogly coupled non-conformal plasma and approach to criticality

Speaker: Panagiotis Betzios
Institution: University of Crete
Time: Tuesday 3 October 2017, 14:15
Venue: 2nd floor seminar room
Abstract: We study fluctuations around equilibrium in a class of strongly interacting non-conformal plasmas using holographic techniques. In particular we calculate the quasi-normal mode spectrum of black hole backgrounds that approach to Chamblin-Reall plasmas in the IR. In a specific limit, related to the exact linear-dilaton background in string theory, we observe that the plasma approaches criticality and we obtain the quasi-normal spectrum analytically. We regulate the critical limit by gluing the IR geometry that corresponds to the non-conformal plasma to a part of AdS space-time in the UV. Near criticality, we find two sets of quasi-normal modes, related to the IR and UV parts of the geometry. In the critical limit, the quasi-normal modes accumulate to form a branch cut in the correlators of the energy-momentum tensor on the real axis of the complex frequency plane.

Primordial Inflation and Power Spectra

Speaker: Nikolaos Tsamis
Institution: University of Crete
Time: Thursday 5 October 2017, 14:15
Venue: 2nd floor seminar room
Abstract: After an introduction to primordial inflation and de Sitter physics we present a formalism which allows to compute primordial power spectra for non-constant geometries analytically and we obtain interesting quantitative results.

Holographic Phonons

Speaker: Matteo Baggioli
Institution: University of Crete
Time: Tuesday 10 October 2017, 13:45
Venue: 2nd floor seminar room
Abstract: We discuss the presence and the physical features of phonons in the context of the holographic framework with particular emphasis on the elastic and transport properties of the dual strongly coupled QFT.

Scale invariant Lifshitz hydrodynamics and Torsional Newton-Cartan geometry

Speaker: Elias Kiritsis
Institution: University of Crete
Time: Thursday 12 October 2017, 14:15
Venue: 2nd floor seminar room
Abstract: Hydrodynamics is the generic low energy theory that describes fluids and generalizes thermodynamics. There are however quantum field theories, whose hydrodynamics is not known. A large class are non-relativistic theories with Lifshitz scaling (appearing in condensed matter systems). The gauge/gravity correspondence has provided a novel and precise correspondence between fluid dynamics and the gravitational physics of horizons. We use this to find the hydrodynamic equations that describe non-relativistic quantum field theories with Lifshitz scaling and hyperscaling violation. We also find the (torsional) Newton-Cartan gravity plays an important role.

Simplifying one-loop amplitudes in superstring theory

Speaker: Dario Consoli
Institution: Universita' di Roma "Tor Vergata"
Time: Tuesday 14 November 2017, 14:15
Venue: 2nd floor seminar room
Abstract: We show that 4-point vector boson one-loop amplitudes in the RNS formalism, around vacuum configurations with open unoriented strings, preserving at least N=1 SUSY in D=4, satisfy the correct supersymmetry Ward identities, in that they vanish for non MHV configurations (++++) and (-+++). In the MHV case (--++) we drastically simplify their expressions. We then study factorization and the limiting IR and UV behaviour and find some unexpected results. In particular no massless poles are exposed at generic values of the modular parameter.

Moduli of AdS4 vacua: A fine-print study

Speaker: Massimo Bianchi
Institution: Universita' di Roma "Tor Vergata"
Time: Thursday 16 November 2017, 14:15
Venue: 2nd floor seminar room
Abstract: We analyze the moduli space near N = 4 supersymmetric AdS4 solutions of string theory by combining and comparing various approaches: - exact backgrounds of type IIB string theory, - gauged supergravity - 3d quiver theories - representations of the superconformal algebra. We discuss both supersymmetry-preserving discrete moduli, and continuous moduli breaking N = 4 to N = 2 supersymmetry. Our analysis lends new support to the underlying holographic duality.

Charting Fundamental Interactions

Speaker: Francesco Sannino
Institution: University of Southern Denmark
Time: Tuesday 13 March 2018, 14:15
Venue: 2nd floor seminar room
Abstract: I will provide a critical overview of the state-of-the-art in particle physics. I will then introduce the recent discovery of novel nonsupersymmetric Wilsonian theories of fundamental interactions of safe rather than free nature. Within the supersymmetric framework I will also present recent exact results for safe super conformal theories and show their impact when searching for theories of grand unification in particle physics. In the light of these new fundamental theories I will address old paradigms and offer novel avenues for (astro) particle physics and cosmology.

Modular Berry Connection

Speaker: Lampros Lamprou
Institution: MIT
Time: Monday 26 March 2018, 13:15
Venue: 2nd floor seminar room
Abstract: States of a CFT's subregions are consistent with a given global state. For a holographic CFT, this amounts to different entanglement wedges being patches of the same geometry. What relations between them make this possible? I will propose a Berry experiment to study this question. Berry introduced a connection to describe transformations induced by adiabatically varying Hamiltonians. I will introduce a connection to study how the zero modes of a modular Hamiltonian are affected by varying the CFT subregion that supplies it. I will explain the geometric meaning of modular Berry phases in the bulk and describe an experiment to measure them which involves observers moving with adiabatically varying accelerations.

Beyond Amplitudes' Positivity and the Space of (In)consistent EFTs

Speaker: Brando Bellazzini
Institution: University of Paris-Saclay
Time: Tuesday 27 March 2018, 14:15
Venue: 2nd floor seminar room
Abstract: In this talk I will discuss which Effective Field Theories (EFTs) may arise from underlying microscopic theories that have an unitarity, analytic, and crossing-symmetric S-matrix. I will show applications in particle phenomenology as well as in cosmology, including recent theoretical constraints on Galileons and Lorentz-invariant massive gravity theories.

Non-supersymmetric dualities and the phases of QCD3

Speaker: Vasilis Niarchos
Institution: Durham University
Time: Thursday 29 March 2018, 14:15
Venue: 2nd floor seminar room
Abstract: A scenario for the phases of vacuum QCD3 has been proposed recently by Komargodski and Seiberg. Three phases occur in this scenario: a phase of bosonization, a phase of spontaneous global symmetry breaking and a phase captured by some IR CFT. I will present an embedding of this scenario in a non-supersymmetric Seiberg duality. The magnetic dual incorporates naturally all the proposed phases. In particular, it incorporates bosonization and symmetry breaking as different patterns of magnetic squark condensation (or in a string theory embedding as different patterns of open string tachyon condensation and brane reconnection). It provides also a natural candidate for the IR CFT of the conjectured CFT phase.

3d Matter Coupled to Chern-Simons Field, Spontaneous Breaking of Scale Invariance and Fermion-Boson Mapping

Speaker: Moshe Moshe
Institution: Technion
Time: Thursday 19 April 2018, 14:15
Venue: 2nd floor seminar room
Abstract: The singlet sector of vector, large N, 3d field theory corresponds to Vasiliev higher spin theory on AdS4. Will discuss three dimensional U(N) symmetric field theory with fermion and boson matter coupled to a topological Chern-Simons term. In the presence of a marginal deformation will determine the conditions for the existence of a phase with spon- taneous breaking of scale invariance. In this phase the ground state contains massive U(N) quanta and a massless U(N) singlet bound state goldstone boson: the Dilaton. Will show that such a phase appears only in the presence of a marginal deformation. The massless Dilaton appears in the spectrum provided certain relations between coupling constants are satisfied. Will discuss the fermion-boson mapping and show that the conditions for spontaneous breaking of scale invariance in the boson and fermion theories are copies of each other.

Cosmological time evolution without time

Speaker: Paolo Benincasa
Institution: Niels Bohr Institutet
Time: Monday 23 April 2018, 14:15
Venue: 1st floor seminar room
Abstract: Given a cosmological observable, how can one check that it comes from a consistent time evolution? In this talk I will report on some recent progress in answering this question and, more generally, understanding cosmological observables such as late-time correlators and wavefunctions of the universe. We provide new techniques for computing them, directly in terms of "boundary data". Strikingly, they have an independent first principle definition in terms of new geometrical objects that we called "cosmological polytopes", which geometrise their singularity structure. Different representations for the wavefunction of the universe can be obtained as different triangulations of the cosmological polytope: This feature allow to find new representations, some of them with no current physical interpretation. Finally, I will comment on the relation between the wavefunction of the universe, the cosmological polytope and the flat-space S-matrix, and how these two observables are related to one another.

Remarks on worldline holography and the static patch

Speaker: Dionysis Anninos
Institution: Amsterdam University
Time: Tuesday 24 April 2018, 14:15
Venue: 2nd floor seminar room
Abstract: We discuss aspects of holography, in the case of a (0+1)-dimensional dual theory. We consider a model where the static patch of de Sitter appears as the infrared region of an asymptotically AdS_2 geometry. We characterize dissipative properties of this state.

Traversable wormholes and the interior of black hole microstates

Speaker: Kyriakos Papadodimas
Institution: CERN
Time: Wednesday 25 April 2018, 14:15
Venue: 2nd floor seminar room
Abstract: Recently it has been shown by Gao-Jafferis-Wall that the eternal AdS black hole/wormhole can become traversable by a time-dependent double-trace coupling between the two CFTs. I will discuss the relevance of this protocol for the study of the interior of 1-sided black holes.

Preheating After Multifield Inflation with Nonminimal Couplings

Speaker: Evangelos Sfakianakis
Institution: NIKHEF, Amsterdam
Time: Thursday 26 April 2018, 14:15
Venue: 2nd floor seminar room
Abstract: Multifield models of inflation with nonminimal couplings are in excellent agreement with the observational results from Planck. It is thus imperative that the preheating behavior of these models is analyzed if one wishes to connect any viable inflationary proposal to the hot Big Bang. I will describe the strong single-field attractor behavior that exists during inflation in these models and how it generically persists after the end of inflation, thereby avoiding the "de-phasing" that can occur in multifield models. This can lead to efficient transfer of energy from the oscillating inflaton field(s) to coupled fluctuations. A doubly-covariant formalism can be used for studying such resonances and identifing several features of preheating specific to the nonminimal couplings, including effects that arise from the nontrivial field-space manifold. I will describe the resonance structure in these models and distinguish three regimes based on the size of the non-minimal coupling: small, intermediate and large. The intermediate and large regimes have the same CMB observables, but drastically different preheating behavior. Finally, I will discuss the extension and application of this formalism to study preheating in other inflationary models.

Holography and thermalization in optical pump-probe spectroscopy

Speaker: Ben Craps
Institution: Vrije Universiteit Brussel
Time: Tuesday 8 May 2018, 14:15
Venue: 2nd floor seminar room
Abstract: Using holography, we model experiments in which a 2+1D strange metal is pumped by a laser pulse into a highly excited state, after which the time evolution of the optical conductivity is probed. We consider a finite-density state with mildly broken translation invariance and excite it by oscillating electric field pulses. At zero density, the optical conductivity would assume its thermalized value immediately after the pumping has ended. At finite density, pulses with significant DC components give rise to slow exponential relaxation, governed by a vector quasinormal mode. In contrast, for high-frequency pulses the amplitude of the quasinormal mode is strongly suppressed, so that the optical conductivity assumes its thermalized value effectively instantaneously.

Quantum criticality gives me butterflies

Speaker: Matteo Baggioli
Institution: University of Crete
Time: Wednesday 9th May 2018, 14:15
Venue: 2nd floor seminar room
Abstract: We study two different aspects of the holographic Weyl semimetal quantum phase transition (QPT). First, we analyze the effects of quenched disorder and we observe the smearing of the QPT, the rare regions effects and the appearance of discrete scale invariance. Second, we probe the onset of quantum chaos, and we find a violaton of the recent conjecture about the butterfly velocity at the quantum critical point. We pinpoint its origin to the anisotropy of the system and we propose a generalized bound which is implied by the bulk Null Energy Condition (NEC).

Quantum chaos, thermalization and black holes

Speaker: Ben Craps
Institution: Vrije Universiteit Brussel
Time: Wednesday 9th May 2018, 17:00
Venue: 3nd floor seminar room
Abstract: Classical chaos refers to exponential sensitivity of phase-space trajectories to small changes in initial conditions. It is not straightforward to extend this notion to quantum systems, which undergo linear evolution and moreover do not exhibit precise phase space trajectories. One possible way to define quantum chaos is via statistical properties of energy levels, which have been related to those of random matrices. Recently, motivated by studies of black holes, exponential growth of out-of-time-order correlators has received a lot of attention as another possible diagnostic of quantum chaos, and an upper bound on the corresponding ''Lyapunov exponent'' has been obtained. Chaos is closely related to the thermalization of isolated quantum systems, for which the current paradigm is the Eigenstate Thermalization Hypothesis. Via gauge/gravity duality, quantum chaos is reflected in the physics of black holes, which are dual to thermal states. In the same way, the study of black hole formation has given a new handle on the thermalization of strongly correlated systems.